This invention relates generally to members utilized for pressure fusing toners at elevated temperatures, and more particularly to improved heat fixing rolls for preventing offsetting of dry toners onto the roll during the fusing operation in electrophotographic copiers.
In the process of electrophotographic copying a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto. Toners are well known in the art and may be of various types.
In order to affix or fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support sheets or members or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support sheet or member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
Several approaches to thermal fusing of electroscopic toner images onto a support have been described in the prior art and include providing the concomitant application of heat and pressure as by a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner takes place when the proper combination of heat, pressure and contact time are provided, the balancing of these parameters being well known in the art and varying according to various factors which must be independently determined for each particular situation.
During operation of a fusing system of the type where there is a thermal fusing of electroscopic toner images onto a support in which at least one fuser member, such as a roll, plate or belt, is heated, the support member to which the toner images are electrostatically adhered, is moved through the nip formed between the members with the toner image pressure contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser member is experienced under normal conditions. This is because the heat applied to the surface of the fuser member is insufficient to raise the temperature of the surface of the member above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting in the molten toner resulting in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such a fuser roll, fuser belt, or fuser plate.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfection in the properties of the surface of the roll; by the toner particles insufficiently adhering to the copy sheet; by the electrostatic forces which normally hold them there; or in certain cases by the reactivity of the toner material itself. In such a case, toner particles may be transferred to the surface of the fuser member with subsequent transfer to the backup member which provides pressure contact, during periods of time when no copy paper is in the nip.
In many of these pressure contact, heat fixing systems, a heated member, e.g. a roll, provided with a covering of a heat-resistant, releasing material on the outer surface thereof is contacted with a back-up or pressure roll covered with a heat-resistant, flexible material layer under nip pressure, between which the sheet to be fixed is passed for fixing the toner image.
As an example of the heat-resistant, release material for the fuser member, there are well known materials such as polytetrafluoroethylene, silicone rubber, fluorocarbon elastomers and the like. In certain cases, a suitable off-set preventing liquid is fed onto the fuser member to minimize or avoid such problems as "offsetting". Silicone oils are widely applied as the off-set preventing liquid.
In the pressure contact heat fixing device of the type described above, the fuser member is covered with a certain thickness of heat-resistant silicone rubber which releases toner material and has an elastic, compressible surface. When silicone rubber is used as the outer layer of the fuser member, the fused image has excellent quality.
The silicone rubbers which can be used as the outer layer coating the base or core of the fuser member, can be classed into three groups according to vulcanization method and temperature, i.e., room temperature vulcanization-type silicone rubber, hereinafter referred to as RTV silicone rubber, low temperature vulcanization-type silicone rubber, hereinafter referred to as LTV rubber, and high temperature vulcanization-type silicone rubber, hereinafter referred to as HTV rubber. These silicone rubbers are well known in the art and are commercially available.
When employed as a fuser member covering material, the RTV and LTV silicone rubbers are generally superior in releasing property to the HTV silicone rubber and thus generally produce favorable results in fusing toner images. The RTV silicone rubbers now commercially available exhibit excellent release of toner images for about 5,000 to 30,000 copies fused by heat. Thereafter, there is a tendency for the off-set problem and winding of copying paper about the fuser roll to occur. When this occurs, it is necessary to replace the used fuser roll.
To overcome the foregoing problems it is well known that silicone oil may be applied to the surface of the fuser member (upon the silicone rubber layer) as an off-set preventing liquid during fixing operations. However, when silicone rubber is used as a heat-resistant, release layer to cover the fuse member, the use of silicone oil is disadvantageous in that the silicone rubber is susceptible to swelling by the action of the silicone oil, so that the oil readily penetrates the rubber, resulting in separation of the rubber from a base or core. Thus, the application of silicone oil is effective in improving the release characteristics, but it sacrifices the durability of the silicone rubber layer covering the fuser member. For this reason, silicone rubber coated fuser members have a short life time and generally permit no more than about 30,000 fusing cycles per fuser member.